Method for Unstacking Veneer Sheets

ABSTRACT

A method for unstacking veneer sheets ( 14 ), in which an uppermost sheet ( 14   a ) of a stack ( 12 ) is taken off the stack by using a suction device ( 24 ) of a movable gripper, and the sheet is then transferred onto a conveyor ( 16 ) and is transported away, wherein the sheet ( 14   a ), after the sheet has been lifted by a first gripper ( 22 ), is transferred to a second gripper ( 32 ), which transfers the sheet to the conveyor ( 16 ).

The invention relates to a method for unstacking veneer sheets, in which an uppermost sheet of a stack is taken off the stack by using a suction device of a movable gripper, and the sheet is then transferred to a conveyor and is transported away.

In the production of veneer laminates, it is frequently required to take off the veneer sheets, which have been superposed on a stack, from the stack (to unstack them) in order to be able to apply an adhesive to each individual veneer sheet, before the sheets are stacked again in several layers and are pressed to form the veneer laminate plate.

It is an object of the invention to provide a robust method for unstacking the veneer sheets.

In order to achieve this object, according to the invention, the sheet, after it has been lifted by a first gripper, is transferred to a second gripper which transfers the sheet to a conveyer.

This method allows in particular the handling of defective veneer sheets which have relatively large holes. In a conventional method, there would be a risk that, when a hole is present in the area of one or more of the suction heads of the suction device, a veneer sheet in the next-lower layer is lifted together with the topmost sheet, so that two sheets, superposed one upon the other, are placed on the conveyer. This leads to substantial complications in the workflow.

In contrast, in the method according to the invention, when the topmost veneer sheet is transferred to the second gripper, the sheet will normally not have another hole in the area where the grip members, e.g. suction heads, of the second gripper attack, and, consequently, these suction heads will only attract and hold to the topmost sheet. Then, when the suction device of the first gripper is deactivated after the sheet has been transferred, the second sheet that had initially been lifted together with the first one will fall back onto the stack. In this way, the veneer sheets can be decollated reliably.

More specific optional features and further developments of the invention are indicated in the dependent claims.

In one embodiment, the method may be configured such that the veneer sheets are lifted by the first gripper and deposited on an intermediate tray from where they are taken off with the second gripper and transferred onto the conveyer.

In another embodiment, the transfer is effected “on the fly”, i.e. the lifted sheet is directly taken over by the second gripper in the state in which it is held in the lifted position by the first gripper.

An embodiment example will now be described in conjunction with the drawings, wherein:

FIG. 1 is a schematic side view of an unstacking apparatus for carrying out the method according to the invention;

FIG. 2 is a front view of the unstacking apparatus shown in FIG. 1;

FIGS. 3 to 6 are views corresponding to FIG. 1, for different stages in an unstacking process; and

FIGS. 7 and 8 illustrate an unstacking operation with defective veneer sheets.

FIG. 1 schematically shows an unstacking apparatus having a support 10 on which a stack 12 of veneer sheets 14 is supported. The support 10 is arranged upstream of one end of a conveyor 16 by which the veneer sheets 14 are to be carried away one by one.

Side frames 18, only one of which is visible in FIG. 1, are arranged on both sides of the support 10 and the conveyor 16. Supported in the side frames 18 are axle studs 20 which carry a first gripper 22 and at least one of which is adapted to be driven for rotation. The first gripper 22 has a first suction device comprising a number of suction heads 26 which, as is shown in FIG. 2, are evenly distributed over the width of the unstacking apparatus. The suction device 24 is held on rockers 28 that are mounted on the axle 20. As is known the art, a gear which has not been shown in detail assures that the suction heads 26 are always facing approximately vertically downwards towards the veneer sheets 14, irrespective of the angular position of the rockers 28. The suction heads 26 are supported flexibly or have flexibility of their own, so that they can adapt to the surface of the veneer sheet 14 to be attracted.

The side frames 18 support another axle 30 that is driven for rotation and carries a second gripper 32 which, in principle, has the same construction as the first gripper 22, comprising a suction device 36 with suction heads 38 mounted on rockers 34. The axle 30 forms part of a slide 40 that is guided in guides 42 in the side frames 18 and is adapted to be moved in the direction parallel to the transport direction of the conveyors 16 by means of a linear drive that has not been shown.

As has been symbolized by stylized springs 44 in FIG. 1, the support 10 is mounted on a lifting device, so that, when veneer sheets 14 are taken off from the stack, the stack is always lifted to such an extent that the topmost sheet will be approximately level with the conveyor 16.

FIG. 1 shows the unstacking apparatus in a state in which the first gripper 22 takes up a topmost sheet 14 a from the stack 12. Accordingly, the suction heads 26 are lowered onto the sheet 14 a.

FIG. 3 shows the unstacking apparatus in a state in which the first gripper 22 has lifted the topmost sheet 14 a. The second gripper 32 is still in stand-by in a lifted position but is then also lowered onto the sheet 14 a in order to take-over this sheet, as has been shown in FIG. 4. During the transfer operation, the two grippers may be stationary, but optionally they may also move synchronously with one another.

As soon as the sheet 14 a is safely held by the suction device of the second gripper 32, the suction device of the first gripper 22 is switched off, so that this gripper releases the sheet, as has been shown in FIG. 5.

Then, the slide 40 is moved in order to draw the sheet 14 a that is held by the second gripper 32 onto the conveyer 16, as has been shown in FIG. 6. Meanwhile, the first gripper 22 returns into the position shown in FIG. 1 in order to take-up a next sheet from the stack 12. The second gripper 32 deposits the sheet 14 a on the conveyor and then also returns to the position shown in FIG. 1 in order for a next cycle to start.

FIG. 7 shows the unstacking apparatus in the same state as in FIG. 2, but illustrates a situation in which the topmost sheet 14 has a hole 46 at a position at which one or more suction heads of the first gripper 22 are located. It should be observed that the thickness of the veneer sheets has been shown exaggerated in the drawings. Thus, in practice, the suction heads 26 of the first gripper 22 and the position of the hole 46 are so close to the next-lower sheet 14 b that they will also attract and lift this sheet. In contrast, the suction device of the second gripper 32 attracts only the topmost sheet 14 a because the suction heads 38 of this gripper are offset from the hole 46.

The grippers 22 and 32 have been arranged such that both grippers are capable of stably holding the lifted sheet, but the spacing between the two rows of suction heads 26 and 38 is larger than the expected largest dimensions of a hole in a veneer sheet. It is therefore extremely unlikely that the suction heads of both grippers attract also the next lower sheet 14 b through a hole in the first sheet.

FIG. 8 illustrates the state in which the second gripper 32 has taken-over the topmost sheet 14 a, whereas the suction device of the first gripper 22 is switched off. Consequently, the sheet 14 b that had initially been lifted as well drops back onto the stack 12, and only the sheet 14 a is transferred onto the conveyor 16 as the unstacking operation proceeds.

In a modified embodiment, the first gripper 22 may also be held at a shaft that is movable in the side frames 18 in longitudinal direction of the conveyor 16 in a range corresponding to the length of the veneer sheets, similarly as the second gripper 32.

This allows to select the grip position as dependent upon the quality of the veneer. 

What is claimed is:
 1. A method for unstacking veneer sheets, comprising the steps of: taking off an uppermost sheet of a stack off the stack by using a suction device of a first gripper, transferring the uppermost sheet to a second gripper, after the uppermost sheet has been lifted by the first gripper, and transferring the uppermost sheet from the second gripper to a conveyor which transports the uppermost sheet away.
 2. The method according to claim 1, wherein the step of transferring includes the step of taking over the uppermost sheet by the second gripper in a state in which the uppermost sheet is held in a position lifted off the stack by the first gripper.
 3. An apparatus for unstacking veneer sheets, comprising: at least one suction device held on a first movable gripper which is arranged upstream of an end of a conveyor and above a stack of veneer sheets, a second movable gripper which is arranged upstream of the end of the conveyor and above the stack of veneer sheets, wherein the first and second grippers are movable independently of one another and wherein movements of the first and second grippers are synchronized such that an uppermost sheet of the stack that has been lifted by the first gripper is adapted to be transferred to the second gripper by switching-off the at least one suction device of the first gripper.
 4. The apparatus according to claim 3, wherein the second gripper has also at least one suction device for holding the uppermost sheet.
 5. The apparatus according to claim 3, wherein the first gripper includes rockers which carry the at least one suction device of the first gripper, and are pivotable about an axis that is fixed relative to the stack.
 6. The apparatus according to claim 3, wherein the second gripper includes rockers pivotable about an axis that is supported in a slide, and wherein the slide is movable relative to the stack in a transport direction of the conveyor. 